Certain semi-crystalline polymers, such as polarized fluoropolymer polyvinylidene fluoride (PVDF) are known to have piezoresponsive properties, which may include piezoelectric response. For this reason, PVDF has been used in various sensors to produce a voltage as a function of force or displacement. Depending upon the structure of a sensor using a piezoresponsive material, and its orientation and the manner of deformation of the piezoresponsive material, a useful response may be developed at electrodes located at various regions of the piezoresponsive material. For example, electrical connections can be made to conductive polymer, metallized foil, or conductive paint laminates or sandwiches containing the piezoresponsive material. The signal produced by such a piezoresponsive material may be in the form of a change in electrical resistance, in the generation of a charge, or the generation of a voltage.
Polymer resin piezoelectric materials are particularly useful because the polymers can be embodied as sensing elements which are both flexible and elastic, and develop a sense signal representing resiliently biased deformation when subjected to force. In the case of PVDF piezoelectric polymer, the sensing element is advantageously embodied as a thin strip. The piezoelectric element is oriented so that the strip is deflected, as by compression or stretching (tension) by the applied force, and two or more electrical contacts are made with the material, so that a voltage signal is produced in response to the force. The voltage is produced because deformation of the polymer material changes the relative positions of charges in the polymer chain or in the semi-crystalline lattice structure. Such sensing elements are useful over a range of frequencies, ranging from near-zero frequencies associated with direct current, up to ultrasound frequencies associated with alternating current. In addition to the sensing of forces, acceleration and displacement, such piezoresponsive sensors can be used in other contexts, such as for the sensing of changes in temperature, or for operation as a switch for generating a trigger signal for operation of a MOSFET or CMOS circuit.
A multi-mode accelerometer is described in U.S. Pat. No. 5,452,612. Another accelerometer is described in U.S. Pat. No. 6,252,335. A rate-responsive pacemaker including an accelerometer-based physical activity sensor is described in U.S. Pat. No. 5,833,713. U.S. Pat. No. 6,252,335 describes a beam accelerometer.
Low-cost cantilever beam type shock sensors are commercially available, as for example the Measurement Specialties Inc. LDTX and LDTM series. Incorporation of these devices into useful products requires the product designer to develop secure and reliable cantilever-clamped/free region boundary conditions so that the conditions at the mechanical boundary between the supported and the free portions of the beam can be predicted and maintained constant from unit to unit. When the electrical response of a piezoresponsive sensor in response to a particular mechanical stimulus may be insufficient, it may be desirable to concatenate together multiple sensors. The concatenation of two or more piezoresponsive sensors into a single unit additionally implicates the problems of achieving the proper electrical interconnections among the individual sensors. Additional problems associated with the design of piezoresponsive sensors lie in the temperature sensitivity of the piezoelectric materials, which may be damaged by overheating attributable to soldering of the electrical connections of the sensor unit to a utilization device. Improved devices are desired.